Such systems are known, for example, from U.S. Pat. No. 4,295,833 and German patent publication no. DE-A1 40 39 425. These systems are employed, for example, in motor boats which exhibit high output and which are difficult to steer without the hydraulic power assist. These power steering systems function according to the principle of force-balance, wherein when the controller is actuated not only is a torque exerted on the control mechanism by way of the existing mechanical connection of the core of the Bowden cable, but also a pressure medium connection is also simultaneously steered to or from the respective desired working chamber of the operating cylinder. In this way the steering force exerted at the controller or the steering wheel is intensified in the requisite direction. The control valve is operated by means of the resulting deformation of the Bowden cable. Following completed adjustment of the control mechanism, the deformation of the Bowden cable is cancelled by way of the controller and the control valve returns to its central position, thus concluding the adjustment of the control mechanism. One particular example of a controller being actuated is the manual operation of a steering wheel or the like.
One drawback of such systems is that, especially when accelerating or decelerating vehicles thus equipped, relatively strong forces can in many cases react on the core of the Bowden cable by way of the control mechanism, starting from the drive unit. Because the sheath of the Bowden cable or the components mechanically connected thereto is prevented from moving with the core of the Bowden cable, which is moved by this reaction, at least by way of seals if not also by way of resetting springs or centering springs for the control valve connected to said components, such reaction-induced adjustments of the power steering system cannot be compensated for. The result is a deviation from the straight ahead movement of the vehicle initiated by way of the acceleration or deceleration of the vehicle and its reaction on the power steering, a state that is normally undesired. In addition, the steering wheel moves thereby, if it is not held tight by hand.